1. Field of the Invention
This invention relates to a slip control system for a vehicle, and more particularly to a slip control system for a vehicle which controls the driving force of the driving wheels when the driving wheels slip due to excessive driving force and controls the braking force applied to the brakes when the wheels tend to lock.
2. Description of the Prior Art
When the driving force of the driving wheels is excessive during acceleration of the vehicle, the driving wheels can slip and the accelerating performance can deteriorate. Thus there has been known a slip control system (traction control system) which, when the slip of the driving wheels relative to the road surface reaches a predetermined value, controls the driving force of the driving wheels so that the slip of the driving wheels converges on a target value. In such a slip control system, the driving force of the driving wheels is controlled by controlling the engine output power and/or applying the braking to the driving wheels.
When the braking force applied to each wheel is excessive during braking, the wheel tends to lock, which can cause skid of the vehicle. Thus there has been known a slip control system (antiskid brake system) which, when the slip of each wheel relative to the road surface reaches a predetermined value, controls the braking force applied to the wheel so that the slip of the wheel converges on a target value.
Further, as disclosed, for instance, in Japanese Unexamined Patent Publication No. 1(1989)-197160, there has been known a slip control system which controls both the driving force of the driving wheels and the braking force applied to each wheel. For the purpose of simplicity, control of the driving force of the driving wheels will be referred to as "traction control" and control of the braking force for preventing lock of the wheels during braking will be referred to as "anti-skid control", and a slip control system which performs both the traction control and the anti-skid control will be referred to as "total slip control system", hereinbelow.
In such a total slip control system, it is important to avoid interference between the traction control and the anti-skid control in order to prevent malfunction of the system. For this purpose, in the total slip control system disclosed in the above identified patent publication, the traction control is inhibited during the anti-skid control.
In view of the safety, it will be preferred that the anti-skid control be performed in preference to the traction control. However if the traction control is interrupted and the anti-skid control is initiated whenever the condition for initiating the anti-skid control is satisfied, the following problem arises.
That is, as a situation where the anti-skid control initiating condition is satisfied during the traction control, there can be supposed a case where the driving wheels are decelerated by engine brake effect in response to a downshift during acceleration. In such a case, slip of the wheels due to the engine brake effect can be suppressed by interrupting the traction control and initiating the anti-skid control. There sometimes happens a situation where the traction control is to be resumed after slip of the wheels is suppressed. Such a situation is apt to occur at a relatively low vehicle speed, and there arises a problem that the controlling accuracy deteriorates as compared with the case where the traction control is continued together with the anti-skid control since various processings required for the traction control such as determination of the friction coefficient of the road surface must be effected again in response to shift from the anti-skid control to the traction control and since it takes some time to end the anti-skid control.